1. Field of the Invention
The present invention relates to a light-source driving device for driving a plurality of light emitting units in an image forming apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus that employs a laser as a light source is widely used for forming an image on a recording medium. Typically, the image forming apparatus includes an optical scanning device that forms an electrostatic latent image on a photosensitive drum by scanning the photosensitive drum with a laser beam in an axis direction of the photosensitive drum via a polygon scanner (e.g., a polygon mirror) while the photosensitive drum rotates. There has been a demand to increase image density, processing speed, and image quality, and improve operability of image forming apparatuses.
For this purpose, a method of scanning a plurality of lines on the photosensitive drum with a plurality of laser beams simultaneously has been proposed.
For example, an image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2000-012973 includes a light emitting element array in which a plurality of light emitting elements, to which a first electrode and a second electrode for applying a current are connected, are two-dimensionally arranged in a rectangular area, and first wires as a row wiring aligned in a longitudinal direction of the rectangular area and second wires as a column wiring aligned in a lateral direction of the rectangular area are arranged in a matrix form. The first electrodes are connected to the first wires, and the second electrodes are connected to the second wires. The light emitting element array is divided into a plurality of groups each of which can be driven independently, the row wiring and the column wiring are provided for the light emitting elements in each group, and the wires in the row wiring are drawn in the column direction.
Moreover, a light emitting element array disclosed in Japanese Patent Application Laid-open No. 2002-314191 includes a plurality of light emitting elements arranged on a substrate and a plurality of electrode pads that are individually connected to the light emitting elements through a plurality of wires provided on the substrate. In the light emitting element array, the stray capacitances of the wires are approximately the same.
In recent years, attention has been paid to the use of a surface-emitting laser element as a light source of an image forming apparatus, such as a vertical-cavity surface emitting laser (VCSEL) element.
For example, a VCSEL element disclosed in Japanese Patent Application Laid-open No. 2002-217488 includes a multiple quantum well (MQW) structure between an active layer and a top mirror, a first electrode for injecting a current into the active layer, and a second electrode for applying an electric field into the MQW structure. The electric field is applied to the MQW structure, so that a refractive index of the MQW structure is changed, thereby making an oscillation wavelength variable. A GaInNAs compound crystal is used as a material of a well layer of the MQW structure in the surface-emitting laser element.
If a light source including a plurality of light emitting units and a driver for supplying a driving current to each light emitting unit are provided on the same substrate and a wiring is conducted between the light source and the driver, it becomes difficult to keep the length of each wire the same between each light emitting unit and the driver as the number of the light emitting units increases.
If the lengths of the wires are different, rise characteristics of lights emitted from the light emitting units differ from each other. In addition, fluctuation in the rise characteristics can arise due to fluctuation in characteristics between the light emitting units, such as thermal characteristics and a resistance.